Nickel supported catalysts for hydrogen production by reforming of ethanol as addressed by in situ temperature and spatial resolved XANES analysis
Graphical abstract
Introduction
The continuous increase of energy demand associated to a larger conscience regarding the environmental problems has been generating a great interest in the development of alternative routes to obtain energy [1]. Hydrogen fuel cells are quite promising systems for energy production, since they have been recognized as sources of energy generation, highly efficient and environmental friendly [2], [3], [4]. Recent research has focused on the production of hydrogen using alcohol and hydrocarbon reforming reactions [5].
Nowadays, most of the hydrogen production in industrial scale is through the process of methane reforming, since methane is the main component of natural gas [6], [7]. However, the development of alternative routes for hydrogen production, using renewable sources, is desirable due to economical and environmental reasons. The ethanol use for hydrogen production is very attractive due to the high content of hydrogen atoms in this molecule [8].
Hydrogen can be produced by steam reforming of ethanol (SRE) (Eq. (1)) and steam reforming of ethanol with addition of oxygen in the feed, called oxy-reforming of ethanol (ORE) (Eq. (2)). However, different parallel reactions occur during reforming of ethanol, as dehydration (Eq. (3)) or dehydrogenation (Eq. (4)) of ethanol and water gas shift reaction (Eq. (5)) [9].
Different catalysts have been studied for the steam reforming of ethanol (SRE) and oxireforming of ethanol (ORE) [9], [10], [11], [12]. Ni- and Co- based materials have been studied for these reactions due to their high capacity for C–C bond scission, compared with the C–O bond scission [13]. However, kinetic equilibrium among the reaction steps is necessary in order to maintain stable catalytic activity [14], because deactivation can be caused by the blockage of active sites due to carbon accumulation, or by the oxidation of active sites [14]. The steam reforming of ethanol over Ni follows a pyrolytic route [15], and the high activity of the metal can result in C–C bond cleavage and subsequent accumulation of carbon on the metal surface. In this case, the presence of O species on the surface is crucial in order to oxidize carbon and reestablish access to the active sites [16]. Deactivation due to carbon accumulation has been widely observed and efforts have been made to develop new catalysts that are more resistant to coke formation [17]. Ni-based catalysts have been studied for both SRE and ORE [17], [18], [19]. However, the catalysts usually present some degree of deactivation. Suppression of carbon accumulation during ethanol conversion reactions is therefore a major issue during catalyst development. Addition of oxygen to the feed enhances the gasification rate of the accumulated carbon and improves the stability of the catalyst [14], but it can also lead to oxidation of the metallic Ni particles, causing a loss of activity in the reforming reactions [20].
The nature of the support strongly influences the catalytic performance of supported nickel catalyst for reforming of ethanol since it affects dispersion and stability of the metal as well as it may participate in the reaction [19], [21]. Thus, the use of different supports can be help to maintain the stability during long reaction periods. Among several different supports tested in the literature, the use of CeO2 and CeZrO2 appears to be a good alternative to keep the catalytic surface free of carbon during the reaction [22]. CeO2 has been used as either an effective promoter or support because of its characteristic oxygen storage capacity, which allows it to store and release oxygen, leading to the presence of highly active oxygen [19], [23]. Additionally, CeO2 also improves the dispersion of active phase [23], [24]. The addition of ZrO2 into CeO2 already has been studied by our research group [24], [25], [26], [27], [28]. It is known that this addition improves the redox properties, oxygen storage capacity, and thermal stability of ceria [29], [30]. Incorporating ZrO2 into CeO2 also promotes water-gas shift, steam reforming, and CO oxidation reactions in the SRE and ORE [23]. Although, ceria–zirconia helps to prevent coke deposition, it usually has higher costs and lower thermal stability than alumina [25].
Oxygen addition to the feed enhances the gasification rate of the carbon deposits improving the catalyst stability [14]. However, it may lead to oxidation of the metallic Ni particles, which can result in activity loss in the reforming reactions [20]. According to literature, the oxygen from the feed can oxidize the surface of catalyst and the presence of metal oxide favors the dehydrogenation of ethanol to acetaldehyde [31]. It is clear that the dependence on the degree of reduction of the metal with the composition of reactants in the feed stream and with the type of support should not be neglected, and it has to be balanced with these requirements to decrease the carbon deposition. It is important to stress that the changes in the oxidation state of Ni have not been investigated under reaction conditions using techniques such as X-ray absorption near edge spectroscopy (XANES). Characterization of the nature of the active sites under reaction conditions is still a challenge, although some progress is now being made.
In this work, we examine the phase changes of nickel catalysts supported on Al2O3, CeO2/Al2O3, CeZrO2/Al2O3 and CeZrO2 in SRE and ORE reactions. Special effort was done to perform in situ characterization by XANES as a function of temperature and reaction conditions. The strong spatial dependence of the oxidation state of Ni along the reactor was also addressed. The experiments aimed to contribute to the elucidation of issues concerning the Ni-catalyzed ethanol reforming reactions such as (i) the sensitivity of stability to the amount of oxygen in the feed stream, (ii) the importance of support nature on catalyst stability and (iii) possible reasons for deactivation.
Section snippets
Catalyst preparation
The samples used in this work were previously described by Dantas et al. [25]. The catalysts were prepared by impregnation of aqueous solutions of nickel nitrate on γ-Al2O3, CeO2/Al2O3, CeZrO2/Al2O3 and CeZrO2 supports. The respective Ni-containing supports were denominate as xNiAl, xNiCA, xNiCZA and xNiCZ, where x is loading in wt% of Ni.
Characterization
BET surface areas, X-ray diffraction measurements (XRD) and Temperature Programmed Reduction (TPR) measurements ex situ results were previously showed by
Light off tests
Fig. 1, Fig. 2 present the values of ethanol conversion and distribution of products as a function of temperature during SRE and ORE, respectively, with W/FEthanol of 2.5 mg min mL−1. During SRE, the 15NiAl sample showed larger ethanol conversion than the samples supported on ceria for temperatures above 673 K (Fig. 1). However, despite all samples presented low activities at lower temperatures (<673 K), the samples with ceria presented higher mole fractions of hydrogen in this condition. At
Conclusions
Overall, the use of different supports affects Ni particle sizes and oxidation states, which influence the rate of carbon accumulation during ORE and SRE. XANES-ORE temperature resolved spectra demonstrate that all samples were oxidized by the feed. The Ni0/Ni2+ ratio is strongly sensitive to the degree of Ni reduction, which also depends on the nature of the support. The control of Ni0/Ni2+ ratio by manipulating temperature, Ni clusters sizes and composition of the feed can equilibrate the
Acknowledgments
The authors wish to acknowledge the financial support of CAPES, CNPq, FAPEMIG, Project VALE/FAPEMIG/FAPESP, FINEP (Financiadora de Estudos e Projetos), Rede Brasileira de Hidrogênio and the Brazilian Synchrotron Light Laboratory (LNLS), which is acknowledged for the use of its facilities and technical support in D06A-DXAS beamline.
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